For youngsters I generally keep them around 80cm. They need to be easily pop over the fences if they are a bit wrong coming into it. Introduce bounces to him with ground poles and small cross poles. They are my bread and butter when it comes to jumping.
Since its a young one its probly just still working out its technique and how to tuck and control those legs so grid work is a good one and risen poles. Make sure its working with enough impulsion on the way into the jump but dont push the horse so its flat, however if it continues you might find he or she is just lacking power or agility in its hind quarters and in that case hillwork will be a good solution :)